Water-CooledChillers

COMMERCIALHVAC CHILLER

EQUIPMENT

Technical Development Programs (TDP) are modules of technical training on HVAC theory, system design, equipment selection and application topics. They are targeted at engineers and designers who wish to develop their knowledge in this field to effectively design, specify, sell or apply HVAC equipment in commercial applications.

Although TDP topics have been developed as stand-alone modules, there are logical group-ings of topics. The modules within each group begin at an introductory level and progress to advanced levels. The breadth of this offering allows for customization into a complete HVAC curriculum – from a complete HVAC design course at an introductory-level or to an advanced-level design course. Advanced-level modules assume prerequisite knowledge and do not review basic concepts.

Water-cooled chillers range in size from small 20-ton capacity models that can fit in an eleva-tor to several thousand-ton models that cool the world’s largest facilities such as airports, shopping centers, skyscrapers, and other facilities. This TDP module will review all sizes of wa-ter-cooled chillers, but will contain more information on the larger chillers in the range of 200-ton and upward. Screw and centrifugal compressor water-cooled chillers tend to be the most popular designs for larger commercial applications, while scroll and reciprocating compressor chillers are used on the smaller ones. Air-cooled chillers are covered in a companion module, TDP-622.

© 2005 Carrier Corporation. All rights reserved. The information in this manual is offered as a general guide for the use of industry and consulting engineers in designing systems. Judgment is required for application of this information to specific installations and design applications. Carrier is not responsible for any uses made of this information and assumes no responsibility for the performance or desirability of any resulting system design.

The information in this publication is subject to change without notice. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, for any purpose, without the express written permission of Carrier Corporation. Printed in Syracuse, NY

CARRIER CORPORATION Carrier Parkway Syracuse, NY 13221, U.S.A.

Table of Contents

Introduction...................................................................................................................................... 1

The First Centrifugal.................................................................................................................... 1 Water-Cooled versus Air-Cooled Chillers................................................................................... 2

Basic Refrigeration Cycle for Water-Cooled Chillers ..................................................................... 4 Subcooling Cycle..................................................................................................................... 5 Economizer Cycle.................................................................................................................... 6

Water-Cooled Chiller Components.................................................................................................. 7 Evaporator.................................................................................................................................... 7

Brazed Plate ............................................................................................................................. 7 DX Shell-and-Tube.................................................................................................................. 7 Flooded Shell-and-Tube........................................................................................................... 8 Evaporator Pros and Cons...................................................................................................... 10 Parallel and Series Chiller Evaporators ................................................................................. 10

Condenser .................................................................................................................................. 11 Compressors............................................................................................................................... 12

Reciprocating ......................................................................................................................... 12 Scroll...................................................................................................................................... 13 Screw ..................................................................................................................................... 13 Centrifugal ............................................................................................................................. 14

Refrigerant Metering – Expansion Device................................................................................. 16 Waterboxes ................................................................................................................................ 17 Purge .......................................................................................................................................... 18 Storage Tank and Transfer (Pumpout) Unit............................................................................... 19 Relief Valves.............................................................................................................................. 20

Chiller Controls.............................................................................................................................. 20 Compressor Starting Methods.................................................................................................... 21

Across-the-Line ..................................................................................................................... 21 Auto Transformer .................................................................................................................. 22 Primary Reactor ..................................................................................................................... 22 Part-Winding.......................................................................................................................... 22 Wye-Delta.............................................................................................................................. 23 Solid-State.............................................................................................................................. 23 Variable Frequency Drive...................................................................................................... 24

Energy Management .................................................................................................................. 25 Chilled Water Reset ............................................................................................................... 25 Demand Limit and Duty Cycling........................................................................................... 25

Screw Compressor Operational Details ......................................................................................... 26 Design and Off-Design Performance......................................................................................... 27

Centrifugal Compressor Operational Details................................................................................. 27 Head ........................................................................................................................................... 28 Lift ............................................................................................................................................. 28 Compressor Boundaries ............................................................................................................. 29 Compressor Stages..................................................................................................................... 31

Capacity Control Methods for Centrifugals and Screws ............................................................... 31 Inlet Guide Vanes ...................................................................................................................... 31 Screw Unloaders ........................................................................................................................ 32 Hot Gas Bypass.......................................................................................................................... 33 Speed Control ............................................................................................................................ 33

Refrigerant Related Topics ............................................................................................................35 Regulations ................................................................................................................................35 Chiller Construction...................................................................................................................36 Safety .........................................................................................................................................37

Heat Transfer .................................................................................................................................38 Heat Balance of Fluid ................................................................................................................39 Overall Heat Transfer.................................................................................................................40 Heat Transfer Coefficient...........................................................................................................41 Impact of Fouling Factor on (U) ................................................................................................42 Impact of Tube Velocity On (U)................................................................................................43 Impact of Tube Material On (U) ................................................................................................44 Evaporator and Condenser Tubing ............................................................................................44 Freezing of Fluids in Tubes .......................................................................................................45 Pass Arrangement ......................................................................................................................45 Variable Flow Operation............................................................................................................46

Codes and Standards ......................................................................................................................46 ARI Testing Standards ...............................................................................................................46 ASHRAE 90.1............................................................................................................................48 UL/CSA & ETL.........................................................................................................................48 ASHRAE Standard 15 ...............................................................................................................49

Selection Criteria ...........................................................................................................................50 Summary ........................................................................................................................................52 Work Session .................................................................................................................................53 Notes ..............................................................................................................................................56 Appendix........................................................................................................................................57

References..................................................................................................................................57 Work Session Answers ..............................................................................................................58

WATER-COOLED CHILLERS

Commercial HVAC Chiller Equipment

1

Introduction This TDP module on water-cooled chillers starts with a history of the first centrifugal chiller

and describes the first applications for early water-cooled chillers. After a discussion of the rela-tive merits of water-cooled chillers, the refrigeration cycle for a water-cooled centrifugal chiller is explained using pressure-enthalpy diagrams.

We will examine the major components used in water-cooled chillers such as evaporators, condensers, compressors, and metering devices. The types of chiller starters and their applications are also discussed.

A greater emphasis is placed on the larger screw and centrifugal types of water-cooled chill-ers in this TDP module. A more detailed discussion of the screw and centrifugal compression process and its characteristics is included. Current refrigerant issues, phase out dates, and appli-cable codes and standards for water-cooled chillers are also examined.

Finally, computerized selection software for a centrifugal chiller is used to demonstrate the required inputs and the selection process for a typical application.

The First Centrifugal

The art of building centrifugal air compressors was already 75 years old in 1916 when Dr. Willis H. Carrier recognized their potential use in the then infant air-conditioning industry.

By 1922, the Carrier Company had purchased a German-manufactured centrifugal air compressor and modi-fied it for use with dielene refrigerant (C2 H2 Cl2). After two years of test and development, this first centrifugal re-frigeration machine was sold in 1924 to the Onondaga Pottery Company in Syracuse, New York. The machine ran for 26 years, providing air condition-ing until 1950. The compressor of that first machine was then retired to the Smithsonian Institute in Washington, D.C. where it remains today on exhibit as one of the major technical develop-ments in the United States.

Dielene

was used in the dry cleaning industry as a cleaning agent.

Figure 1 Early Centrifugal Chiller

WATER-COOLED CHILLERS

Commercial Chiller Equipment

2

Carrier’s second centrifugal ma-chine was installed in 1923 in Cambridge, Massachusetts, at the candy manufacturing plant of the W. F. Schraft and Sons Company. The second one built ended up being in-stalled prior to the first one.

The first centrifugal machine

was tested for two years at Carrier. It was sold to the Onondaga Pottery Company in Syracuse, New York.

Because of those early efforts in the 1920s, water-cooled chillers have gained widespread acceptance in both large and medium systems. Technology has re-sulted in the evolution of water-cooled chillers, which are characterized by their excellent reliability, high efficiency, and compact, cost-effective construction.

Water-Cooled versus Air-Cooled Chillers

Two methods are used to con-dense the refrigerant in chillers. The condensers can be air-cooled or wa-ter-cooled. A typical air-cooled chiller uses propeller fans to draw ambient air over a finned coil to condense the refrigerant. It may contain multiple or single compressors. For a complete discussion on air-cooled chillers, refer to TDP-622, Air-Cooled Chillers.

Figure 2 A Carrier chiller is on display at the Smithsonian Institute.

Figure 3 Evolution of Centrifugal Chillers

Figure 4 Air-Cooled and Water-Cooled Chiller Benefits

WATER-COOLED CHILLERS

Commercial HVAC Chiller Equipment

3

A typical water-cooled chiller uses recirculating condenser water from a cooling tower to condense the refrigerant. For a complete discussion on cooling towers, refer to TDP-641, Con-densers and Cooling Towers.

Cost and efficiency are the important factors when considering air or water-cooled chillers. Chilled-water systems with air-cooled chillers typically have lower in-stalled and maintenance costs than water-cooled because a condenser water system using a cooling tower is not required. A condenser water pump and chemical treat-ment for the condenser water loop adds to the maintenance required with a water-cooled system. How-ever, water-cooled chillers have higher efficiency and

therefore lower operational costs. Air-cooled chillers are chosen when it is impractical to use a cooling tower, such as when little water is available or water is highly corrosive.

The refrigerant condensing temperature in an air-cooled chiller is dependent on the ambient dry-bulb temperature. In a water-cooled chiller, refrigerant condensing temperature is dependent upon the entering condenser water temperature (and flow rate), which is a function of the ambient wet-bulb temperature. Since the wet-bulb temperature is always lower than the dry-bulb tempera-ture, the refrigerant condensing temperature (and pressure) in a water-cooled chiller is often significantly lower than in an air-cooled chiller. This is why water-cooled chillers are more efficient.

In terms of capacity, air-cooled chillers are available in packaged sizes ranging up to approximately 500 tons, while water-cooled chillers are typi-cally available up to 3,000 tons, with limited custom designs available up to 10,000 tons.

Water-cooled chillers typically last longer than air-cooled chillers. Air-cooled chillers may last 20 years while water-cooled chillers may last 23 years or more. This may be attributable to the fact that water-cooled chillers are installed indoors, and most air-cooled chiller configurations spend their lives outdoors in the elements. Also, some of the larger water-cooled chillers are con-structed with heavy duty, industrial-grade components.

Absorption Chillers

This TDP will cover water-cooled chillers using the vapor-compression refrigeration cycle. Absorption chillers that use water as the refrigerant will not be covered in this TDP.

Figure 5 Typical Water-Cooled Chiller System

WATER-COOLED CHILLERS

Commercial Chiller Equipment

4

Basic Refrigeration Cycle for Water-Cooled Chillers In this TDP, we will explain the refrigeration cycle using components from a centrifugal

chiller since that type of chiller is water-cooled. The following temperatures are typical of the standard refrigeration cycle for comfort cooling applications. In the evaporator of a water-cooled chiller, liquid refrigerant at ap-proximately 42º F takes on heat from building return water (whose entering temperature may be represented at 54º F) flowing through the evaporator, and changes to a vapor. The re-frigerant vapor is drawn into the compressor and its temperature and pressure are elevated. The compressor provides the work necessary to compress the gas to a temperature and pressure re-quired by the condenser, typically 97º F. The gas is then discharged into the condenser where it condenses on tubes through which water flows, typically at 85º F. This is the entering condenser water from the cooling tower. The condensed droplets of liquid refrigerant then fall to the bottom of the condenser, flow through a pressure reducing de-vice such as a float valve or an orifice, and return to the bottom of the cooler where the process repeats itself.

The cycle can be shown on a pressure-enthalpy (p-h) diagram. Pressure is the force exerted per unit area, while enthalpy is the total heat content expressed in Btu per pound of the substance. When the compressor is close-coupled to the evaporator, there is negligible pressure loss in the suction line, and gas enters the com-pressor at approximately the saturated conditions that exist in the evaporator, Point 1.

If we follow the steps shown, you can see that from A to 1 is the refrig-eration effect. In this step, building heat from the chilled water is ab-sorbed by the refrigerant, and the refrigerant transitions from a liquid to a saturated vapor, at Step 1. From Step 1 to 2 is the com-pression stage. This stage raises the temperature and pressure of the saturated refrigerant vapor to the saturated condensing temperature, so that heat can be rejected to the condensing fluid. This compression is also called the “lift” of the compressor, which will be discussed later in the TDP.

Figure 6 Components of a Centrifugal Chiller Refrigeration Cycle

Figure 7 Pressure-Enthalpy (p-h) Diagram Showing Lift